In many agricultural applications, implements are to be guided in a constant position above the ground of a field or above the plant grown thereon. Examples include harvester heads in front of harvesting machines, which must be guided at the most constant possible cutting height in order to cut off the plants at a defined height above the ground, or field sprayers, which must be guided across the field at the most constant possible height above the leaf canopy of the plant growth in order to apply sprayed material to the plants from a defined height. Maintaining a constant position relative to the transport vehicle, even when the harvesting machine travels over uneven ground, also makes sense for height-adjustable transfer devices with which crop is transferred from a harvesting machine to a transport vehicle, since the constant height can prevent loss of crop.
The position of the harvester head for harvesting machines is usually adjusted by actuators that pivot an intake housing of the harvesting machine along with the harvester head mounted on the front side thereof about a horizontal axis oriented transversely to the forward direction. For combines, this pivot axis typically coincides with an upper deflection roller of the intake conveyor in the intake housing, and for forage harvesters, it typically coincides with the axis of rotation of the chopper drum.
In the simplest case, the height of the harvester head above the ground is detected by sensors in the form of sensor bars or the like that are mounted below the harvester head. The output signals of the sensors are fed to a control unit that adjusts the actuator to adjust the position of the harvester heads for maintaining a desired cutting height (e.g., European Patent Application No. EP 0 331 893 A2), another actuator adjusting the harvester head relative to the harvesting machine about an axis oriented in the forward direction in order to align the harvester head parallel with the ground.
Such arrangements are subject to the disadvantage that the position of the harvester head also depends on the vertical position of the ground engagement means (tires or track drives) of the harvesting machine. If the front wheels drive over an obstacle (i.e. a bump on the ground) the harvester head is necessarily moved upward, and if the rear wheels drive over the obstacle, the harvester head is necessarily lowered. The monitoring arrangement for adjusting the position of the harvester head cannot respond directly to the movement of the harvesting machine, however, but learns of this only from the sensor values for the harvester head. Due to the response time of the actuators, a prompt response is not always possible in all cases, however, which can cause the harvester head to penetrate into the ground in some circumstances, with the result that dirt is picked up along with the crop, or even that the harvester head is damaged. A manual override by the operator in such cases is provided in the prior art according to European Patent Application No. EP 0 331 893 A2.
It was additionally proposed to detect the ground profile in front of the harvesting machine prospectively with a non-contact sensor using laser beams (e.g., European Patent Application No. EP 1 269 823 A1) or to connect a non-contact sensor or a sensor with ground contact to the harvester head (e.g., European Patent Application No. EP 1 356 729 A1 and German Patent Application No. DE 10 2010 040 872 A1) and to use the output signal thereof for timely enough activation of the actuator that vertical movements of the harvesting machine caused by unevenness of ground can be compensated by the monitoring arrangement. Such an arrangement can also be based on a stored topographical map of the field, which is queried location-specifically when harvesting (e.g., International Publication No. WO 2008/088916 A2).
When driving the actuator, however, the prospective monitoring arrangements mentioned in the preceding paragraph do not take account of the fact that the system consisting of ground, tires and harvesting machine is not incompressible in all cases. In other words, the monitoring arrangements assume that the uneven ground is not deformed (i.e. is incompressible), that the tires do not deform and that there is no suspension between the ground engagement means and the harvesting machine, so that there is no vertical relative motion between the ground profile and the harvesting machine. In reality, these assumptions are not accurate, however, because the shape of the uneven ground, at least that of loose sandy soil, will change (to some extent reversibly and to some extent irreversibly) when driven over by a combine with a full grain tank, which can hold 10,000 liters. The tires of a harvesting machine are also not incompressible, but instead deform when moving over ground deformations. Furthermore, machines are sometimes equipped with hydro-pneumatically suspended front wheels (e.g., German Patent Application No. DE 199 00 212 A1), hydro-pneumatically suspended track drives (e.g., International Publication No. WO 2006/018215 A1) or sprung rear wheels (e.g., German Patent No. DE 200 07 795 U1), which cannot be considered rigid systems per se. When driving over uneven ground, movements (particularly vibrations) thus arise in the tires and/or suspensions that are also transmitted via the harvesting machine and the intake housing to the harvester head and cannot be taken into consideration by conventional monitoring arrangements for positioning the harvester.
European Patent No. EP 2 583 545 A1 proposes to detect the deformation of the tire with a sensor. This deformation is used to calculate a compensation signal, on the basis of which the height at which the harvester head would be situated if the tire were no longer deformed is calculated. Based on this signal, the height of the harvester head is adjusted by the actuator in such a manner that the desired height above the ground is achieved. This therefore does not compensate for the vibration arising when traveling over uneven ground, but instead compensates for the fact that the varying position of the harvesting machine relative to the ground due to the deformation of the tire was not previously taken into consideration because the cutter height measurement was solely based on the sensor for determining the height of the cutter above the ground. A compensation of the vibration in the tire when driving over uneven ground is not possible with the above prior art arrangement.